A variety of knock control fluids have been developed to mitigate abnormal combustion events, including various combinations of gasoline, ethanol, methanol, other alcohols, water, and other inert fluids. Water injection, for example, reduces knock, provides charge cooling, and reduces the octane requirement. In addition, since water injection can also be used for engine dilution control, the need for a dedicated knock control fluid is reduced.
Another example of a knock control fluid is shown by Surnilla in U.S. Pat. No. 7,533,651. Therein, direct injection of a washer fluid, which includes water and alcohol (e.g., engine coolant or methanol) leverages the charge cooling properties of the both the fluid and the direct injection to reduce knock. In addition to protecting the water from freezing, the inclusion of engine coolant in the composition of the injected knock control fluid offers an added advantage of having light hydrocarbons (such as methanol), which help in the combustion process. The overall approach increases engine efficiency while reducing the octane requirement of injected fuel, thereby increasing the power output of the engine. Herein, the wiper fluid can be repurposed for knock control in addition to being used for cleaning a vehicle windshield.
However, the inventors herein have recognized an issue with the approach. There may be variations in windshield wiper fluid composition. For example, there may be a wide variation in the ethanol or methanol content of the fluid, as such. In addition, when a windshield wiper fluid tank is refilled, based on an amount and composition of wiper fluid that was left over in the tank, the composition of the available wiper fluid following the refilling may vary. While this does not affect the fluid's ability to clean a windshield wiper, it may affect the knock controlling ability of the fluid. For example, the octane value of the fluid may change. As such, various engine parameters are adjusted based on the injected knock control fluid. For example, based on the alcohol content of the injected fluid, cylinder fueling may be adjusted. In addition, engine parameters may need to be adjusted based on the type of alcohol in the fluid (e.g., whether the alcohol is ethanol or methanol). As a result, errors in the estimation of a wiper fluid composition can result in significant air-fuel errors, degrading engine performance. Further, if the composition of a wiper fluid is not accurately known, use of wiper fluid as a knock control fluid may be limited. On the other hand, the addition of a sensor dedicated to estimating the alcohol content and composition of a knock control fluid may add significant cost and complexity. While an intake oxygen sensor may be used to estimate the alcohol content of the knock control fluid during selected conditions, the inventors have recognized that the presence of diluents such as humidity, positive crankcase ventilation (PCV) hydrocarbons, and purge gases can cause errors in the output of the intake oxygen sensor, corrupting the estimation.
In one example, the issues described above may be addressed by a method for an engine comprising: injecting a water-alcohol blend into an engine intake initially at a first flow rate and then at a second, different flow rate; applying a reference voltage to an oxygen sensor and monitoring a change in pumping current of the sensor following each injecting; learning a first portion of the change in pumping current due to a water content of the blend; learning a second portion of the change in pumping current due to an alcohol content of the blend; and learning a third portion of the change in pumping current due to diluents in intake air. In this way, the composition of a knock control fluid injected into an engine can be accurately determined using an existing oxygen sensor, such as an intake or an exhaust oxygen sensor, with noise factors being mitigated.
As an example, following refilling of a wiper fluid tank, a wiper fluid composition may be estimated using an intake oxygen sensor. The wiper fluid may then be used as a knock control fluid. As such, the wiper fluid may include a mixture of water and alcohol but no gasoline. Further, an alcohol type in the fluid may be known a priori. For example, it may be known that the wiper fluid is a water-ethanol mixture, or a water-methanol mixture. However, a ratio of water to the specified alcohol in the fluid may not be accurately known. A controller may first inject the knock control fluid at a first gaseous volume percent into the intake manifold, downstream of an intake throttle and upstream of an intake oxygen sensor. The fluid may be injected while EGR is disabled to reduce interference on the results from EGR. A lower reference voltage (e.g., 450 mV) may then be applied to the intake oxygen sensor and an output of the sensor may be noted. For example, a first pumping current may be output. The controller may then inject the knock control fluid at a second, different gaseous volume percent into the intake manifold. The different volume percentages may be provided by injecting different amounts of the fluid at a given air mass flow rate, or by injecting a given amount of the fluid at different air as flow rates. The lower reference voltage (e.g., 450 mV) is then reapplied to the intake oxygen sensor and a second output of the sensor may be noted. For example, a second pumping current may be output. As such, the pumping currents may be affected by a reduction in the oxygen concentration at the oxygen sensor due to the water content of the knock control fluid as well as due to the alcohol content of the knock control fluid, and further due to the presence of diluents, such as PCV gas hydrocarbons, in the intake air. Specifically, the water in the knock control fluid may have a dilution effect on the oxygen sensor while the alcohol in the knock control fluid and the diluent hydrocarbons in the intake air may combust with oxygen at the sensor, reducing the oxygen concentration estimated at the sensor. An engine controller may normalize the first and second pumping currents based on the corresponding injection mass flow rates and then learn a common offset in the first and second pumping currents due to the presence of diluents in the intake air. After adjusting for the diluents, the controller may calculate the alcohol content of the knock control fluid based on the first and second normalized pumping currents, as well as the injection mass. For example, the engine controller may reference a 3D calibration map to estimate the alcohol content of the fluid, and update the composition of the fluid. By learning the composition of the fluid, the flexibility of usage of the wiper fluid as a knock control fluid may be enhanced.
It will be appreciated that while the above example discusses washer fluid composition estimation using an intake oxygen sensor, in alternate examples, the estimation may be performed using an exhaust gas oxygen sensor (such as a UEGO). Therein, the fluid may be injected at the first and second gaseous volume percent during deceleration fuel shut-off (DFSO) conditions and the composition may be determined based on a change in the pumping current of the UEGO.
In this way, an existing intake or exhaust oxygen sensor can be used to estimate the composition (including the hydrocarbon type and alcohol content) of a knock control fluid while compensating for corruption of results by intake air diluents. The technical effect of monitoring the output of an intake oxygen sensor after injecting fluid at different volume percentages is that a change in the pumping current of the sensor that is attributed to the water component of the knock control fluid can be better distinguished from the change attributed to the alcohol component of the knock control fluid without requiring adjustments to EGR, purge or PCV flow to reduce noise. This is due to the fact that the dilution effect on the oxygen sensor has a remarkably different contribution than the combustion effect of the alcohol. In addition, a common diluent based offset may be learned because the diluents cause a similar relative change in the pumping current at the different volume percentage rates. By better estimating the composition of an injected knock control fluid, the use of the knock control fluid may be expanded to engines of different fuel types, improving the robustness of the system. In addition, the accuracy of fuel octane estimates may be increased, which allows spark control to be improved. For example, spark retard usage for knock control may be reduced providing fuel economy benefits. By learning the noise in the pumping currents cause by the presence of intake air diluents, the knock control fluid can be estimated accurately even when purge or EGR is present. By using an existing intake oxygen sensor to determine the composition of the knock control fluid, the need for a dedicated sensor is reduced without compromising on the accuracy of the estimation.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.